When circulation of blood, and therefore oxygen, to muscle is interrupted (ischemia), the ability of the muscle to contract is impaired. Even if circulation is restored (e.g., by reperfusion), muscle function can remain depressed. In certain types of muscle, such as cardiac muscle, the effects of ischemia can have severe consequences, and inadequate circulation of blood to the heart is one of the most important causes of morbidity in developed countries. Clinically, ischemia and reperfusion injury manifest as a spectrum. In its mildest form, ischemia is transient, reperfusion is established quickly and the reduced contractility of the muscle tissue is temporary and reversible. However, longer and more severe ischemia produces irreversible damage and cellular necrosis.
The functions of myofilament specific proteins, such as troponin I and troponin T (members of the regulatory complex), myosin light chain 1 (MLC1), and α-actinin are affected during hypoxemia (i.e., reduced delivery of oxygen due to a reduced partial pressure and/or arterial content of oxygen), ischemia, and/or ischemia/reperfusion injury. These changes affect contraction of muscle by apparently altering the interaction of troponin I, troponin T, myosin light chain 1, and α-actinin with other proteins critical for normal muscle contraction.
Troponin I is a key component of the troponin regulatory complex which directly controls striated (cardiac and skeletal) muscle contraction and relaxation. Troponin T is also part of the troponin complex and is involved in assembly of troponin-tropomyosin on the actin filament. α-actinin is a cytoskeletal protein, the main component of the Z lines. Traditionally α-actinin was believed to keep the actin filaments aligned. However, the large changes in conformation of Z lines during the cross bridge cycle suggest that α-actinin has a dynamic role during muscle contraction. Myosin light chain 1 is an integral part of the myosin myofibril. Myosin light chain 1 is found in slow and fast skeletal and atrial and ventricular cardiac muscles.
To date, the underlying molecular changes responsible for the reduced contractility of injured muscles resulting from hypoxemia, hypoxemia, ischemia, and reperfusion are not known. As a result, early diagnosis of the above-mentioned states and assessment of the extent of muscle damage in a subject, particularly the difference between reversible and irreversible muscle damage, has not been possible. The current serum diagnostic indicators used in diagnosing myocardial infarction (e.g., anti-troponin I, anti-troponin T and anti-creatine kinase MB antibodies by Spectral Diagnostics Inc., Toronto, Canada; anti-troponin T, anti-MLC1 antibodies by Baxter Inc., Chicago, Ill.; and anti-troponin I antibody by Baxter Inc.) are indicators of myocardial necrosis (irreversible damage) since they detect proteins which are released from the heart myocyte following the loss of cellular membrane integrity. These indicators provide no information as to the extent or type of myocardial damage, or the molecular nature of which would clearly benefit the art. To date there are no commercially-available serum or urine markers for skeletal muscle damage.